1. Field of the Invention
The present invention relates to a synchronous motor with permanent magnets which is suitable for operation under equivalent field weakening control, and a motor system including such a synchronous motor.
2. Description of the Relevant Art
BASICS AND APPLICATIONS OF BRUSHLESS SERVOMOTORS, published by General Electronic Publishing Company, describes the technology given below, in Chapter 6, Section 2, Control of Brushless Servomotor, and Chapter 6, Section 4, Equivalent Field Weakening Control.
Alternating currents flows through a brushless servomotor, and the relative speed of axes (d-, q-axes) rotating in synchronism with the alternating currents is zero, simplifying an equation model.
According to a model based on the d/q-axis conversion proposed by Coulomb, the voltage and current of a brushless motor are related to each other as described by the equations (1) below. ##EQU1## EQU Te=3/4.times.(number of poles).times..phi.id=J.omega.m+B.omega.m+Tl
where
Ra: Armature resistance, PA1 La: Armature inductance, PA1 J: Motor inertia, PA1 .phi.: Magnetic flux (constant) of permanent magnets, PA1 .omega.m: Motor angular velocity, PA1 B: Coefficient of viscous friction, PA1 p: d/dt, PA1 Vd: d-axis voltage, PA1 Vq: q-axis voltage, PA1 id: d-axis current, PA1 iq: q-axis current, PA1 Te: Output torque, and PA1 Tl: Load torque.
Generally, the q-axis current iq is controlled so that it is zero. Therefore, the d-axis current id becomes the motor current. Since the values pLa id and pLa iq are zero under the normal condition, the voltages Vds, Vdq at the terminals of the motor are expressed by the following equations (2): EQU Vds=Ra id-.omega.mLa iq+.omega.m.phi., EQU Vdq=Ra iq+.omega.mLa id (2).
The field of a brushless servomotor with permanent magnets cannot be controlled because the field current cannot be varied. The voltage of the armature increases in proportion to the speed of the rotor. When the armature voltage exceeds the maximum voltage of a voltage source PWM inverter, the motor cannot operate at a speed higher than the speed corresponding to that voltage.
The equivalent field weakening control process which reduces the field intensity based on the armature reaction makes it possible to rotate the motor at higher speed.
According to the torque equation (1) and the equations (2), if the q-axis current iq is iq=0, then the motor terminal voltage increases in proportion to the motor angular velocity .omega.m and the d-axis current id, i.e., the instantaneous torque. Therefore, the equivalent field weakening control process is carried out by controlling the q-axis current iq.
FIG. 1 of the accompanying drawings shows voltage vectors under the normal condition.
In FIG. 1, V0 represents the motor terminal voltage when the q-axis current iq is iq=0.
When the q-axis current iq is fully controlled, the motor terminal voltage V can be equalized to the maximum output voltage Vmax of the voltage source PWM inverter, as shown in FIG. 1.
The motor current should be lower than an allowable maximum current Imax. Therefore, the maximum output torque should be limited in its magnitude when the motor operates at high speed.
FIG. 2 of the accompanying drawings shows current vectors indicating the path a motor current i follows when the output torque of the motor is maximum. FIG. 3 of the accompanying drawings shows the relationship between the torque, the output, the voltage, and the motor speed that are required for the motor to operate under the field weakening control.
If the motor angular velocity .omega.m exceeds a value .omega.m1 (FIG. 3), then the motor terminal voltage V is held to Vmax by controlling the q-axis current iq. The maximum output torque is reduced by reducing the d-axis current id.
If the motor angular velocity .omega.m exceeds a value .omega.m2, then the q-axis current iq is held to Iqmax.
If the motor angular velocity .omega.m exceeds a value .omega.m3, then the d-axis current id, i.e., the output torque Te, becomes zero. The motor angular velocity .omega.m3 at this time is the allowable maximum rotational speed of the synchronous motor with permanent magnets.
Various motors have been operated according to the equivalent field weakening control process. As a result, it has been found that the motors have widely different ranges of rotational speeds that can exceed the output (hereinafter referred to as "rated output") at the maximum rotational speed (hereinafter referred to as "rated motor speed") under normal operating conditions, the ranges of rotational speeds being representative of a ratio k by which the rated motor speed can be multiplied into the rotational speed.